I don't particularly believe running the Game in compatibility is the solution or answer as his OS is 64 bit and Crysis was made for it but it would be something to try since the OP did not really give us a generous explanation of the problem.
Things I would try, upgrading my Video, Monitor and Sound drivers
to the latest versions as sometimes when installing games, drivers may become corrupted.
Crysis was a very intensive CPU/GPU hog and it brought high-end rig to its knees. Heat may be an issue here as well, check temps.
Are you running dedicated PSU wires for your GTX275?